1. Field of the Invention
The present invention relates to a method of producing an SOI wafer having an SOI (Silicon On Insulator) structure in which a silicon layer is formed on an insulator, and to an SOI wafer produced by use of the method.
2. Description of the Related Art
In these years, an SOI wafer having an SOI structure in which a silicon layer (SOI layer) is formed on an insulator has been especially attracting attention as a wafer for a high-performance LSI for a electronic device because the SOI wafer is excellent in high-speed property, low-power consumption, high breakdown voltage, environmental resistance etc. for the device.
Representative production methods of the SOI wafer are SIMOX method in which an oxide film is formed in a silicon wafer by subjecting, to a heat treatment at a high temperature after implanting oxygen ion into the wafer at a high concentration, a method called bonding method, etc. The bonding method is a method of producing an SOI wafer in which an SOI layer is formed on a buried oxide film being an insulator, by, after forming an oxide film on at least one of a bond wafer to form an SOI layer and a base wafer to be a supporting substrate and bonding the bond wafer with the base wafer through the oxide film(s), making the bond wafer into a thin film.
Known as production methods of the SOI wafer utilizing the bonding method are grinding and polishing method, PACE (Plasma Assisted Chemical Etching) method, ion implantation delamination method (also called “Smart Cut” (registered trademark) method, see Japanese Patent Application Laid-open Pub. No. 05-211128), ELTRAN method, etc (see “Science in Silicon”, published by Realize publishers, pp. 443-496).
Here, the ion implantation delamination method will be described with reference to FIG. 2. First, two silicon wafers of a base wafer 11 and a bond wafer 12 are prepared (Step (a′)). Next, after forming an oxide film 13 on at least one of these wafers (in this case, the bond wafer) (Step (b′)), an ion-implanted layer 14 is formed inside the bond wafer 12 by implanting at least one kind of ion of hydrogen ion and a rare gas ion into the bond wafer 12 (Step (c′)). Then, after the ion-implanted surface of the bond wafer 12 is bonded with the base wafer 11 through the oxide film 13 (Step (d′)), the bond wafer 12 is delaminated at the ion-implanted layer 14 as a cleavage plane (the delaminating plane) by subjecting to a delaminating heat treatment (Step (e′)), thereafter, an SOI wafer 17 in which an SOI layer 16 is formed on a buried oxide film 15 can be produced by subjecting further to a boding heat treatment for strengthening the bonding between the wafers, a mirror polishing process called “touch polishing” in which polishing stock removal is very small and other processes (Step (f′)).
However, when producing an SOI wafer, once a mirror polishing process including a mechanical element has been carried out at the final stage as described above, a problem that the uniformity of the thickness of the SOI layer achieved by the ion implantation and delaminating is degraded occurs because the polishing stock removal is not uniform. Furthermore, since the mirror polishing is performed after the bonding heat treatment, the method has many steps and is complicated and disadvantageous also in terms of cost.
In order to solve such a problem, Japanese Patent Application Laid-open Pub. No. 11-307472 for example discloses a technique in which, after wafers have been bonded to each other and a bonding heat treatment has been carried out for them, a high-temperature heat treatment is carried out in hydrogen or Ar atmosphere in order to reduce the surface roughness and crystal defects of the SOI layer of an SOI wafer without carrying out any mirror polishing.
Furthermore, with the higher integration of semiconductor devices in these years, production of a higher-quality SOI wafer is sought and, for example, an SOI wafer having a thinner buried oxide film and an SOI wafer for which crystal quality of its SOI layer is improved are sought.
Generally, when an SOI wafer is produced in the ion implantation delamination method as described above, in order to form a buried oxide film having a desired thickness in the SOI wafer, the SOI wafer is produced by forming an oxide film formed on at least one of a bond wafer and a base wafer such that the thickness of the oxide film is the same as a desired thickness of the buried oxide film and, thereafter, bonding these wafers to each other.
However, when producing an SOI wafer having a thin buried oxide film having a thickness of, for example, 100 nm or less, after wafers are bonded to each other, in carrying out a delaminating heat treatment, as shown in FIG. 3, there are often cases where a blister 34 and a void 35 are generated in the SOI wafer in which a buried oxide film 32 and an SOI layer 33 are laminated on a base wafer 31 and unbonded portions are caused. Then, there has been a problem that, as the thickness of the buried oxide film of the SOI wafer becomes thinner, these blister and void tend to be generated more, and good wafers become more difficult to obtain and the yield becomes worsened.
From now on, the thickness of a buried oxide film formed in an SOI wafer is expected to proceed in such a course as it is reduced from 100 nm to 80 nm, further, 50 nm or less. Therefore, it is desired to produce an SOI wafer at a high yield without generating any blister and any void even when the thickness of the buried oxide film is reduced.